[etherlab-users] Maximum mailbox size?

[Dave Page]

On 2014-07-08 11:21, Gavin Lambert wrote:
> Though one of my original questions remains:
>>> If it's not supposed to work, shouldn't the master just select a
>>> smaller size if it doesn't support large sizes?  While it's supposed
>>> to respect the slave's preference, ultimately the master is in control
>>> of mailbox sizes.
>
     I don't know.

     A brief look at the SSC suggests the master is responsible for 
specifying the mailbox size each transfer via the Length parameter in 
the mailbox header. The FMMU mailbox protocol requires the last mailbox 
byte to be accessed to hand over the mailbox buffer. So, the fieldbus 
must transfer the full mailbox size (or utilize two transfers?).

     So, based on that alone, it would appear the master could specify a 
smaller mailbox size so long as the mailbox header Length is set correctly.

     Obviously, a 1486 byte mailbox would result in very little room for 
process data. But the master is free to send multiple ethernet frames, 
so as long as the bus utilization is not exceeded, that does not appear 
to be a problem.

     It would be interesting to hack the master to override the mailbox 
size read from the SII to a small fixed value (e.g. 123) and then see 
what breaks.

         Best regards - Dave




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